The present invention is concerned with an electrical drive mechanism for drills, cutting tool and screw drivers which are employed, customarily, for surgical purposes. The invention also concerns a process for the modulation of the drive mechanism and for the monitoring of the state of the driven tool.
In the field of medicine, operational apparatuses of this kind, are used especially in dentistry for common root canal preparatory work, and also for boring and cutting threads into bone structure, which would include borings for insertions such as rods. In addition, these operations are carried out manually by the dentist or an operator, in order to avoid damage to the teeth or to the bone being treated.
For instance, in dentistry, normal root canal preparations are carried out by means of an electrically driven drill, which is seated in a dental hand piece. Generally, a non-commutator, direct current drive is employed, such as is marketed by the firm KaVo under the designation xe2x80x9cINTRAmatic LUXxe2x80x9d. This type of drive, which is usually dedicated for application in dental drilling operations, is driven, within the scope of dentally approved control in the lower range of 2000 to 4000 RPM up to, in the highest rotational speed range, about 4000 to 60,000 RPM and at a torque of 2 to 3 Ncm. These drive units are incorporated within the dental hand piece.
For root canal preparatory work, however, flexible endo-tools are used, by means of which root canals, which can exhibit varied diameters and different paths, are cleaned out. This is carried out, for instance, with a spiral shaped, flexible drill, tapering toward the point and of a length of approximately 20 to 30 mm. This device, by means of its turning, screws itself into the root canal, whereby, upon the withdrawal thereof, the canal is reamed out. According to the manufacturer of this kind of a drill, the rotation speed of the same runs in a range of about 100 to 500 RPM. In exceptional cases, this might be as high as 1800 RPM. Then, in order to achieve the optimal operative range for this tool, motors selected from those now available, must be inserted into the conventional hand piece. These motors, with the previously named speeds of rotation of 2000 to 4000 must possess in the drive train, a speed reducer of i=16:1 to 20:1. By means of drives of this kind, indeed the applicable area of known electric motors for surgical purposes has been extended to include the drives for spiral drill of the described, generic kind. However, the speed reducer brings into activity a torque increase of exactly the above said ratio, reduced by the efficiency loss of the drive.
Investigations by the inventor have provided the results, that with this torque, nearly all of the endo-tools being used at this time, for instance for root canal preparatory work, are stressed above their breaking threshold. For instance, it has been demonstrated, that flexible spiral drills for the cleaning of root canals may only be loaded up to a maximum of 0.2 Ncm. If this torque threshold be overstepped, the spiral drill can break off and remain implanted in the root canal being treated. Accidents of this type, in many cases, can only be alleviated by an operative measure.
Thus, in the face of this problem, the purpose of the invention is to create an electrical drive mechanism for surgical endo-tools, the speed of rotation and torque ranges of which are variably adjustable by equipment of minimal control technological complexity. Further the purpose includes that the electrical drive mechanism can be employed by nearly all of the existing endo-tools, especially for drilling and also for cutting threads and the engaging of rods therein.
The invention of the present patent is comprised accordingly in the design of an electrical drive mechanism for tools for surgical drilling, cutting and screw thread related functions, with, respectively, a predetermined range of speeds of rotation and a maximum allowable torque loading threshold. The mechanism further encompasses an electrical stepping motor, the maximum torque and RPM of which can be preset by the magnitude of the current and the frequency of the rotational field. Extensive experiments brought forth the result, that with a stepping motor as a drive for contemporary endo-tools, the breakage of said tools is prevented in their practical applications. In accomplishing this, one is making use of the effect of xe2x80x9cfalling out of stepxe2x80x9d, which, when pushed to the highest degree, results in a blockage of the motor upon overload. The regulatory technical complexity remains at a minimal level and limits itself to the adjustment of the ampere load as well as the pulse generation for the stepping motor.
A particular goal of the invention is that the above described mechanism must be adaptable for different hand pieces and thus for different drive shaft appurtenances, which again, exhibit varying degrees of efficiency. Expressed in other words, the maximum torque generated by the motor in a hand piece does not match the maximum torque applied directly to the tool. The invented procedure for the corrective compensation of the drive mechanism provides a self-calibration step, which is carried out in operation following an exchange of the hand piece or hand grip carrying the drive train. This said step, however, can be executed also at designated time periods. Principally, in this matter, the invented stepping motor is loaded with a predetermined minimal electrical current below the initial start-up current level. Subsequently, the amperage is increased incrementally up to the point where the motor starts to run. This final amperage value, which activates a start of the stepping motor, is saved in memory as that current, which is required for overcoming the friction of the drive shaft train and which, as a result, is also proportional to the current consumed by the drive chain.
The electrical drive mechanism is thereby improved in that a drive shaft of the stepping motor is coupled to the tool with or without an up or down ratio transmission. Under these circumstances, the stepping motor again with/without an up or down transmission, possesses an operational rotational speed range of from 100 to 300 RPM and drops out of step upon a loading equal to, or greater than its adjusted preset torque.
The operational rotational speed range is also the start-stop-RPM spread of the stepping motor, within which the said motor, after dropping out of step, upon a lessening of the torque loading, can recover itself and start to run again. It is advantageous if the stepping motor, even outside of its said start-stop-RPM, can be run up to a rotational speed of 6000 RPM.
The above described development thus enables a more extensive use of an electrical drive with tools having a low fracturing load and brings about a substantial lessening of the danger of break-off of the tool. This also opens the possibility of multiple usages of the same tool. Furthermore, it has become evident, that for endo-tools, which are subjected to a continually increasing time of operation, the failure-point loading thresholds, determined by the inventor in the meantime, are subject to a corresponding change. Within the context of tests, a maximum operational time could be determined analytically for some endo-tools, within which the likelihood of breakage is held down and the cutting ability of the tool is sufficient. Upon exceeding these maximum usage periods, it is noted that breakage as a result of material fatigue is seen to increase and as well, a substantial deterioration of the cutting performance becomes evident.
Thus, to make full use of the ability to operate at full capacity as well as maintain the safety potential of the newly developed drive concept of the inventor, the most exact knowledge about the condition of the tool in use is necessary, in order to prevent a break-off in every case, not only as a result of an impractical overload, but also as a result of material fatigue.
In view, then, of the problems presented, it is technically advantageous to make available a new procedure for recognizing the condition of a surgical tool, in particular, of an endo-tool.
In keeping with the invention, a procedure for the monitoring of the condition of an endo-tool, which at least encompasses the following steps:
a. the input to a computer of the previously determined, maximum allowable loading quantity, of a specific tool,
b. the determination of a partial loading quantity resulting from an actual treatment and the addition of this said partial loading quantity to a total loading quantity specific to the tool condition, which total quantity results from previous treatments,
c. a comparison of the maximum allowable loading quantity with the up-to-date total loading quantity and
d. the emitting of a xe2x80x9cNeed to Exchangexe2x80x9d signal, in case the entire total loading quantity reaches the loading threshold or exceeds the same.
By means of the foregoing, described procedure, there will be subsequently assigned to a tool, for instance, a flexible spiral drill, a maximum loading quantity for which the risk of break-off is minimal and on a continuing basis, the xe2x80x9cCondition Nowxe2x80x9d loading quantity is computed and compared with the maximum loading quantity. In this manner, a sufficiently safe condition of the tool can be assured.
In accord with claim 15, the loading quantity is defined as a theoretical amount, which is defined by the running rotational RPM speed, the torque, and the length of the surgical treatment. Additionally, or alternately, the number of sterilization cycles can serve as a value to be compiled in the total loading quantity.
Additional advantageous embodiments of the invention are, in this matter, subjects of the subordinate claims.